1. Field of the Invention
The present invention relates to the calibration of communication channel parameters in systems, including mesochronous systems, in which two (or more) components communicate via an interconnection link; and to the calibration needed to account for drift of conditions related to such parameters during operation of the communication channels.
2. Description of Related Art
In high-speed communication channels which are operated in a mesochronous manner, typically a reference clock provides frequency and phase information to the two components at either end of the link. A transmitter on one component and a receiver on another component each connect to the link. The transmitter and receiver operate in different clock domains, which have an arbitrary (but fixed) phase relationship to the reference clock. The phase relationship between transmitter and receiver is chosen so that the propagation delay seen by a signal wavefront passing from the transmitter to the receiver will not contribute to the timing budget when the signaling rate is determined. Instead, the signaling rate will be determined primarily by the drive window of the transmitter and the sample window of the receiver. The signaling rate will also be affected by a variety of second order effects. This system is clocked in a mesochronous fashion, with the components locked to specific phases relative to the reference clock, and with the drive-timing-point and sample-timing-point of each link fixed to the phase values that maximize the signaling rate.
These fixed phase values may be determined in a number of ways. A sideband link may accompany a data link (or links), permitting phase information to be passed between transmitter and receiver. Alternatively, an initialization process may be invoked when the system is first given power, and the proper phase values determined by passing calibration information (patterns) across the actual link. Once the drive-timing-point and sample-timing-point of each link has been fixed, the system is permitted to start normal operations.
However, during normal operation, system conditions will change. Ambient temperature, component temperature, supply voltages, and reference voltages will drift from their initial values. Clock frequencies may drift due to environmental and operational factors, or be intentionally caused to drift in spread spectrum clock systems, and the like. Typically, the frequency drift will be constrained to lie within a specified range, and many of the circuits in the components will be designed to be insensitive to the drift. Nonetheless, the drift will need to be considered when setting the upper signaling rate of a link. In general, a channel parameter may be calibrated as a function of one or more changing operating conditions or programmed settings. In many cases, drifting parameters will be plotted in the form of a two-dimensional Schmoo plot for analysis. Examples of programmed settings, which might be subject of calibration, or which might cause drift in other channel parameters, include transmitter amplitude, transmitter drive strength, transmitter common-mode offset, receiver voltage reference, receiver common-mode offset, and line termination values.
As the conditions drift or change, the optimal timing points of the transmitter and receiver will change. If the timing points remain at their original values, then margin must be added to the timing windows to ensure reliable operation. This margin will reduce the signaling rate of the link.
It is desirable to provide techniques to compensate for the condition drift, and provide improvements in system and component design to permit these techniques to be utilized.